For wireless communications, the channel conditions may vary significantly both in time and in frequency. The variation in time relates typically to how fast the transmitter and the receiver are moving, but may also to some extent relate to movements in the environment. The variations in frequency may typically relate to the channel's impulse response and in particular to the delay spread of the channel. When the delay spread of the channel is large, the channel variation in frequency is larger than if the delay spread is small.
If the bandwidth of the signal is large, e.g. 20 MHz, then the channel will typically vary considerably over the bandwidth, whereas if the bandwidth of the signal is relatively small, e.g. 1 MHz, the channel may often be approximated as being frequency flat within the bandwidth of the signal.
The numerical values used above to classify a signal as wideband and narrowband respectively are representative when the distance between the transmitter and the receiver is rather short, typically in the range of 10 to 100 meters. A situation where this is applicable is for instance for indoor communications using Wi-Fi. In cellular systems, where the distance between the transmitter and the receiver may be several kilometers, the experienced channel may vary significantly within, e.g. a bandwidth of 100 kHz.
In order to determine whether a channel should be considered as frequency flat (typically a narrowband channel) or frequency selective (typically a wideband channel), it is typically common to relate the root mean square (RMS) delay spread of the channel to the symbol rate, which in turn can be approximated as the same as the bandwidth of the signal. A channel can then typically be assumed to be frequency flat if the RMS delay spread of the channel is less than 10% of the symbol duration.
As an example, if the symbol rate is 1 Msymbols/s and the RMS delay spread is 50 ns, the channel can typically be assumed to be flat since the symbol time Ts=1 μs and thus the RMS delay spread is only 5% of the symbol duration.
A narrowband channel is typically smaller than the coherence bandwidth (i.e. the maximum range of frequency where the channel response is flat), and smaller than the wideband channel which typically exceeds the coherence bandwidth.
The terms wideband and narrowband are relative, and the sizes of the bands may typically be in kHz, MHz or GHz depending on the type of application used.
A frequency flat channel may typically have the advantage that the receiver may be constructed without the need for a channel equalizer. On the other hand, one typical disadvantage is that the entire channel may be bad, i.e., the entire channel may simultaneously be experiencing weak signal conditions due to fading.
The latter issue, i.e., that the channel is experiencing bad signal conditions due to fading, may for instance be counteracted by changing the used frequency on a regular basis. This is the idea behind frequency hopping. The change of frequency may for instance be between packets, like in Bluetooth Wireless Technology, but it can in principle also be between symbols.
In any event, applying frequency hopping typically leads to that the wireless link will not experience fading conditions constantly or for longer periods of times, and therefore the system performance will typically depend on the average (over the frequencies) channel performance rather than the worst case conditions.
One simple means to obtain frequency diversity is by using dual carrier modulation (DCM). In DCM, the same information is typically transmitted on two resource units (RUs), and the receiving side then combines the information received on the two carriers.
In order to not degrade the transmission rate, the amount of information sent on each one of the carriers used for DCM is doubled. As an example, an access point (AP) operating in a Wi-Fi network is to send information to two wireless terminals operating in the same network, and binary phase shift keying (BPSK) is used for both wireless terminals. If DCM is not used, then the available bandwidth could typically be shared between the two terminals such that the first terminal is allocated to the lower frequency part of the channel whereas the second terminal is allocated to the upper frequency part of the channel. However, with DCM, the information is instead typically repeated for the two terminals such that the same information is sent on both the lower and the upper part of the band. However, in order to achieve this, quadrature phase shift keying (QPSK) is typically used instead of binary phase shift keying (BPSK).
Although the reception of signals transmitted by means of DCM will typically be slightly more complex, there may be a significant gain due to the increased frequency diversity.
However, one of the basic ideas of DCM is typically that the receiver is able to receive both carriers and then combine them. If DCM is used when the receiver of a terminal is only able to receive one of the sub-carriers, there typically arises a significant risk of inferior reception due to data losses.
A situation where the receiver is only able to receive one of the carriers may e.g. occur when the receiver is part of a very inexpensive and power efficient implementation (e.g. a LRLP receiver). These types of implementations are typically expected to increase. A terminal having a LRLP receiver may typically be denoted as a narrowband terminal i.e. a terminal operating in a limited frequency range, typically such that the frequency range is frequency flat.
A wideband terminal, on the other hand is typically operating in a wide frequency range, where the frequency range is typically frequency selective. The wideband terminal is typically further equipped with a transceiver capable of receiving on multiple carriers.
Hence there exists a problem in that DCM in its basic form is not well suited for being used with narrowband (LRLP) receivers.
In “A single-antenna dual-carrier selection technique for frequency selective fading channels” by Jingli Li, Gang Zhao and Xiangqian Liu, a diversity technique for single-antenna systems in frequency-selective channels is presented. Assuming that the channel band is divided into multiple subchannels, symbols are transmitted on two frequency hopped subchannels simultaneously. To keep the total transmission power constant, the signal power in each of the two subchannels is reduced to half of what would be in single channel transmission. At the receiver selection diversity is employed by decoding the subchannel with the larger gain. Furthermore, channel diversity can be obtained by combining complex-field coding (CFC) with the dual carrier selection design.
However, dual carrier selection demands large spectrum resources as the used spectrum is doubled, and the paper does not provide a solution for how the receiver may determine which channel has the largest gain.
Therefore, there is a need for wireless terminals and access points that enables reliable wireless communication regardless of transmission and reception abilities.